In charged-particle-beam (CPB) microlithography (especially such microlithography as performed using an electron beam as a representative charged particle beam), proximity effects are an annoying problem. According to conventional methods, certain features in the reticle pattern are configured such that, when projected onto the substrate, the exposed features have interior non-exposed areas having dimensions that are smaller than the resolution limit of the microlithography apparatus. Certain other areas of the reticle pattern, that are not exposed when projected onto the substrate, have interior exposed areas having dimensions that are smaller than the resolution limit of the microlithography apparatus. Such conventional methods reportedly provide a degree of correction of proximity effects, especially in electron-beam microlithography. Jpn. J. Appl. Phys. 34:6672-6678, 1995.
Unfortunately, the above-summarized technique loses effectiveness whenever the feature density is not uniform within an "exposure unit" (i.e., the area of the reticle exposed at any instant of time). I.e., non-uniform space-charge effects can increase to troublesome levels whenever the reticle comprises exposure units having different feature densities. Such space-charge effects cause inconsistent beam defocusing due to differences in the focal position resulting from the space-charge effects in the high-density feature areas versus in the low-density feature areas. Space-charge effects can also introduce concave-lens effects that result in deformation of projected features.
Furthermore, conventional techniques are not effective in countering the so-called resist-heating effect in which resist development differs substantially from pattern areas having relatively small features versus pattern areas having relatively large features.